Glucose-based polysaccharides and their derivatives can be of potential industrial application.
Cellulose is a typical example of such a polysaccharide and is comprised of beta-1,4-D-glycosidic linkages of hexopyranose units. Cellulose is used for several commercial applications such as in manufacture of fibers and films (cellophane). Cellulose for industrial applications is derived from wood pulp. Solutioning of wood pulp is a difficult procedure. For cellophane production the most commonly used process for dissolution of cellulose is the ‘viscose process’ where the cellulose is converted to cellulose xanthate made by treating a cellulose compound with sodium hydroxide and carbon disulfide. The cellulose xanthate solution is extruded into a coagulation bath, where it is regenerated upon coagulation to form a cellulose film. Cellophane film has several desirable attributes like clarity, barrier to oxygen, mechanical strength etc which has resulted in its application as a packaging film. However, the disadvantage is the use of this viscose process in cellophane manufacture, which involves toxic chemicals and significant environmental costs. In addition, cellulose films show poor moisture resistance. Cellulose-derivative films, specifically cellulose acetate films, are used when moisture resistance is required. A widely used process for the preparation of cellulose acetate as described in U.S. Pat. No. 2,478,425 A comprises (1) a pretreatment step (activating step) of mixing a cellulose material having a high α-cellulose content with a small amount of an acid, (2) an acetylating step of treating the pretreated cellulose material with a mixed acid of acetic anhydride, acetic acid and an acidic catalyst, such as sulfuric acid, to obtain primary cellulose acetate, (3) a ripening step of hydrolyzing, according to need, the primary cellulose acetate obtained by the acetylation step to obtain cellulose acetate or cellulose acetate having a higher acetylation degree and (4) a purifying step of separating and purifying the obtained cellulose acetate by precipitation, solid-liquid separation, washing and drying. This process has some significant concerns as described in U.S. Pat. No. 4,306,060. First, large energy requirements for cooling the reaction—the acetylation reaction is exothermic but the reaction should be carried out at a temperature lower than room temperature. Second, during the ripening step, a part of the acetic acid ester connected to the cellulose portion of primary cellulose acetate is hydrolyzed. During this step, however, the ether linkage of the cellulose main chain is liable to be broken by hydrolysis. This tendency is prominent when sulfuric acid, added as the acetylation reaction catalyst, is coupled with cellulose in a large amount, and an excessive reduction of the degree of polymerization results. In order to isolate sulfuric acid bonded to cellulose, while preventing reduction of the degree of polymerization, and to effect hydrolysis to a desired degree of acetylation, the hydrolysis is ordinarily conducted for a very long time at a temperature slightly higher than ambient temperature, but lower than 40° C. Third, a cellulose starting material having a very high quality and a high α-cellulose content should be used. In the case of wood pulp, there is a quality standard for the acetate grade wood pulp.
Films of cellulose acetate can be prepared by either by melt extrusion methods or by casting methods. For many reasons, however, films prepared by melt extrusion are generally not suitable for optical applications such as for protective covers and substrates in electronic displays. Rather, casting methods are almost exclusively used to manufacture films for optical applications. Casting methods involve first dissolving the polymer in an appropriate solvent to form a dope having a high viscosity, and then applying the viscous dope to a continuous highly polished metal band or drum through an extrusion die, partially drying the wet film, peeling the partially dried film from the metal support, and conveying the partially dried film through an oven to more completely remove solvent from the film.
Amongst polysaccharide polymers, glucan polymers, with alpha-1,3-glycoside linkages, have been shown to possess significant advantages. U.S. Pat. No. 7,000,000 disclosed preparation of a polysaccharide fiber comprising a polymer with hexose units, wherein at least 50% of the hexose units within the polymer were linked via alpha-1,3-glycoside linkages, and a number average degree of polymerization of at least 100. A glucosyltransferase enzyme from Streptococcus salivarius (gtfJ) was used to produce the polymer. The polymer alpha-1,3-glucan was acetylated in order to render the polymer soluble in the spinning solvent. The acetylated polymer was then dissolved in a mixture of trifluoro-acetic acid and dichloromethane. From this solution continuous, strong, fibers of glucan acetate were spun. These glucan acetate fibers can subsequently be de-acetylated to form fibers composed of alpha-1,3-glucan.
It would be desirable to make films composed of a polysaccharide alpha-1,3-glucan polymer which have properties comparable to cellophane, without the need for acetylation and subsequent de-acetylation. In addition, elimination of the use of hazardous chemicals such as carbon disulfide required for xanthation of cellulose would be desirable. In addition, it would be desirable to make films with properties comparable to cellulose acetate, without the need for a separate acetylation step.